The investigators' study involves 1 000 patients, treated in the Rhône-Alpes / Auvergne region. The patients present an indication for colonoscopy following a positive fecal occult blood test or symptoms or personal and family histories of pre-cancerous colonic lesions. They will be randomized into two study groups:one of both groups of study: Group 1: patients undergoing a colonoscopy wtih instillation of tepid water tinged with blue (Indigo Carmin®) Group 2: patients undergoing a standard colonoscopy with insufflation of air. The main objective is to show that the colonoscopy with blue water (Indigo Carmin®) enable detection of more lesions than standard colonoscopy with insufflation of air. The two techniques will be also compared in terms of tolerance, of duration of colonoscopy and type of detected lesions.